This invention is related to the field of electrochemical cells or electrochemical batteries, more particularly to rechargeable lithium batteries.
Most electrochemical cells include a negative electrode, a positive electrode and an electrolyte providing passage for the ionic electroactive species of the electrochemical cell. Electrolytes may be solid or liquid or a composite of both. The electrodes are usually prevented from coming into direct contact by some form of a separator or solid electrolyte, which allows the movement of ionic electroactive species but not of electrons. Electrochemical cells or batteries are usually equipped with current collectors which can be connected to an external electrical circuit for utilizing the electrical energy generated by the battery. In case of rechargeable electrochemical cells or batteries, the same current collectors serve in recharging the battery or cell.
In the last decade or more lithium batteries have been developed for generating electrical energy. Rechargeable lithium batteries may be cylindrical or button shaped and in such formats they often have a non-aqueous liquid electrolyte. More recently, thin plate rechargeable lithium batteries have been developed which are suitable for use in electronic devices of current design, as well as having high energy density per volume or weight. Rechargeable thin plate lithium cells or batteries most often utilize as the anode active substance, lithium foil or lithium alloy, or a substance capable of reversibly intercalating lithium ions. The cathode of a rechargeable lithium battery usually contains a transition metal chalcogenide or equivalent, as the positive active material. The electrolyte of a thin plate rechargeable lithium battery may be a solid electrolyte laminate containing lithium ions, or a separator sheet in which a non-aqueous solution containing the electroactive component, that is a compound bearing a dissociable lithium ion, is dispersed. Separators for lithium batteries are frequently formed of inert porous or microporous polymer layers or sheets, which are subsequently impregnated with a liquid electrolyte containing a dissolved lithium salt or similar substance. The polymer sheet either as a solid electrolyte or as host for a liquid electrolyte, needs to be durable and strong to render effective barrier between the electrodes, as well as to be able to supply sufficiently high concentration of mobile electroactive species per unit area for yielding high current density. It can be seen that the development of suitable electrolytes is a very important aspect of thin film rechargeable lithium battery technology.
Conventional solid polymer electrolyte compositions incorporate dissociable lithium ion bearing compounds in their structure. The mobility of the electroactive species in the polymer matrix will depend on the nature of the lithium compound having labile lithium ions, as well as on the temperature of the lithium battery operation and such like. It is noted that the mechanical strength of polymers capable of incorporating dissociable lithium ion bearing compounds is often low and may also be subject to degradation by the electrode materials if the temperature of the battery rises above normal operating temperatures. The lack of mechanical strength may require that solid polymer electrolytes have substantial thickness, which may lead to diminished energy density per unit volume for lithium batteries. The ionic resistance of lithium ion conducting solid polymer electrolytes are usually in the range of 10xe2x88x924 to 10xe2x88x922 S/cm.
Hybrid electrolytes for thin plate rechargeable lithium batteries often utilize organic solvents or mixtures thereof for the dissolution of a lithium compound. There are known solvents or mixtures of solvent compounds, such as disclosed, for example, in U.S. Pat. No. 5,643,695 issued to Barker et al. on Jul. 1, 1997. As briefly referred to above, an hybrid lithium battery electrolyte has an inert porous separator layer for keeping the electrodes separated and to hold in its pores and micropores a large reservoir of dissociable lithium ions for enabling the lithium battery to generate high current density. The lithium battery may be assembled of a negative electrode layer, a positive electrode layer and an inert plasticised separator layer between the electrodes. The plasticizer may be, at least in part, replaced by an organic lithium ion solution before packaging the battery, as is described in U.S. Pat. No. 5,456,000, issued to Gozdz et al. on Oct. 10, 1995. Inert polymer separators composed of multiple layers of polyolefin membranes of different porosity and melting point, are described in U.S. Pat. No. 4,650,730, issued to Lundquist et al. on Mar. 17, 1987. It is noted that most known separator sheets are inert, in other words, only the electroactive components of the organic solution retained in the cavities of the separator layer take part in the cell reaction. High pore density of the separator sheet may provide a high population of electroactive species but it may also undermine the mechanical strength, and hence the durability of the hybrid electrolyte.
More recently composite hybrid electrolytes for use in rechargeable lithium batteries have been described, wherein the separator is impregnated and/or coated with an inert gel of organic, polymerizable composition. Such multi-layered polymer systems are described in U.S. Pat. Nos. 5,681,357, 5,688,293 and 5,716,421, issued to Eschbach et al., Oliver et al. and Pendalwar et al, on Oct. 28, 1997, Nov. 18, 1997 and Feb. 10, 1998, respectively. In the multi-layered polymer systems for use in lithium batteries the inert porous polymer separator is a polyolefin layer and the polymerizable gel is polyvinylidene fluoride (PVDF) or chemically equivalent polymer or copolymer. The gelling compound as described in the above publications, is supported by the porous polyolefin layer, and is intended to serve as an inert absorbent for the lithium ion containing organic solutions which is added subsequently. In the methods taught by Eschbach et al., Oliver et al. and Pendalwar et al. the gelling compound is cured and polymerized in the packaged and sealed battery by subjecting the package to heat and pressure, thus also bonding the electrodes to the composite separator. The heat and pressure treatment which is required to solidify the gelling compound of the lithium batteries made according to the above methods, may damage the packaging of the lithium battery so produced, thereby rendering the packaging more vulnerable to moisture and similar atmospheric damage. Moreover, the curing of the battery components subsequent to packaging and sealing may generate undesirable gases and similar compounds detrimental to the satisfactory operation of the lithium battery. It is also noted, that in the multi-component polymer electrolyte systems containing gelling compounds, there is only one kind of electroactive species present, which is added to the multi-component electrolyte subsequent to assembling the the electrochemical cell.
There is a need for an electrolyte system for use in thin plate rechargeable lithium batteries which provides enhanced mechanical integrity and strength, as well as capability of high ionic conductivity without unwarranted increase in the thickness of the electrolyte layer.
A new composite electrolyte has been found for use in thin plate rechargeable lithium batteries, comprising an inert porous or microporous first polymer laminate layer carrying a microporous or porous layer or coating of a second polymer on at least one of its major faces. The second polymer layer is containing a dissociable lithium compound and the at least two polymeric layers are forming a composite structure. A portion of the pores or micropores of the first polymer layer is filled with the second lithium compound bearing polymer in the composite structure. The composite porous structure is subsequently impregnated with a lithium salt bearing non-aqueous organic liquid.
The composite electrolyte is placed between the negative and positive electrodes of a rechargeable lithium battery, thereby forming a thin plate rechargeable lithium battery which can be packaged with appropriate current collectors.